My rebarreled Ballard #4 in .45-70 has a 1-18" Green Mountain barrel, that is 34" long. As John mentioned, if you plan on using a longer barrel, such as a 32-34", I would go for the 1-18". Mine shoots everything from 300 grain to 550 grain bullets, with equal accuracy. I do however only shoot cast bullets in mine, and never more than 1500 fps. My favorite loads are usually about 1350-1400 fps.
Hope this helps.